The present invention relates to light modulators and more specifically to light moudlators employing surface waves as their source of modulation.
Acoustic bandwidth modulation of light has been achieved using two basically different techniques, volume diffraction of a light beam by bulk acoustic waves (Bragg diffraction), and surface reflection or diffraction.
Volume diffraction techniques employ a medium whose refractive index can be varied in accordance with the modulating signal. In U.S. Pat. No. 3,938,881, a transducer generates an acoustic strain wave which alters the refractive index of an acousto-optic medium. Light incident upon the medium near the Bragg angle is modulated according to the strain wave. However, bulk acoustic wave Bragg diffraction modulators designed to be operated at high power efficiency do not develop sufficient spatial modulation bandwidth.
Surface modulation of light occurs when light is modulated as it is reflected from a surface to which the modulating signal has been transmitted. Various techniques have been used to transmit this signal to the reflective surface. In one such method, the surface is excited by the direct acoustic pressure of an incident sound wave. A second method employs solid surface modulations produced by piezoelectrically excited Rayleigh waves. Both of these techniques require impractically large acoustic power levels to achieve significant bandwidth at high diffraction efficiency.
In another surface modulation technique, a free liquid surface is excited by radiation pressure. This is a quasi-static effect associated with continous, finite amplitude sound waves. However, free surface modulation produces only narrow temporal modulation bandwidth and the devices employing it cannot be fabricated to be rugged.
All of these reflective surface devices, when designed to operate over wide spatial bandwidths at temporal bandwidths below one megahertz are too large to be practical.
Recent attempts to improve surface reflection light modulators have been directed toward improving modulators that use a mirrored membrane as their reflective surface. These devices use an electric signal carrying modulation information to electro-statically deflect the membrane. In U.S. Pat. No. 3,796,480 improvement of the localized deflection of the membrane is disclosed. This modulation and others like it do not modulate with surface waves.